1. Field of the Invention
The present invention relates to the processing of substrates, such as semiconductor wafers, using photolithography. More specifically, the present invention relates to methods for correcting photoresist-induced spherical aberration that occurs during photolithography.
2. State of the Art
Semiconductor devices including integrated circuitry, such as memory dice, are mass produced by fabricating hundreds or even thousands of circuit patterns on a single semiconductor wafer or other bulk semiconductor substrate using photolithography in combination with various other processes. In order to increase the number of memory cells on semiconductor memory devices for a given surface area, it is important to accurately control the optical resolution of the images produced during photolithography. These images are used to define structural features on a semiconductor substrate to fabricate the integrated circuitry of such semiconductor memory devices.
Photolithography is a process in which a pattern is delineated in a layer of material, such as a photoresist, sensitive to photons, electrons, or ions. In photolithography, an object containing a pattern (e.g., reticle or mask) is exposed to incident light. The image from the reticle or mask is projected onto a photoresist that covers a wafer or other substrate. The photolithographic process typically involves exposing and developing the photoresist multiple times. At a given step, the photoresist is selectively exposed to photons, electrons, or ions, then developed to remove one of either the exposed or unexposed portions of photoresist, depending on whether a positive or negative photoresist is employed. Complex patterns typically require multiple exposure and development steps.
There are three predominant conventional photolithography methods of optically transferring a pattern on a mask to a photoresist that is coated on a substrate. These methods are contact printing, proximity printing, and projection printing. Currently, projection printing is the most frequently used type of exposing system. Referring to FIG. 1, a conventional exposing system used in projection printing is shown. Exposing system 100 includes illumination controller 101 coupled to illumination source 102 for producing light. Illumination source 102 typically includes a mirror, a lamp, a light filter, and/or a condenser lens system. In the exposing system shown in FIG. 1, illumination source 102 irradiates mask 108 having the desired pattern to be projected onto photoresist 110. Projection lens 104, which may include a complex set of lenses and/or mirrors, focuses the image from mask 108 onto photoresist 110. Photoresist 110 is developed and substrate 112 is subsequently processed as by etching to form the desired structures and photoresist 110 is then removed.
When projecting a mask pattern onto a photoresist using the above-described process, it is typically necessary to compensate for spherical aberration induced by the exposing system. Spherical aberration is caused by using spherically shaped lenses and mirrors because truly spherical surfaces do not form sharp images. An example of a spherical aberration is shown in FIG. 2. Paraxial light rays 202 and peripheral light rays 204 are incident on a lens 206. Paraxial light rays 202 and peripheral light rays 204 do not unite accurately at a focus. Rather than the light being focused at a point, it is focused along a line transverse to the plane of the target photoresist, resulting in a blurred image. The spherical aberration is defined as the distance between focal point 210 of paraxial light rays 202 and focal point 208 of peripheral light rays 204. By using a complex system of lenses and/or mirrors, the spherical aberration of an optical system used in conventional photolithography may be, and typically is, adjusted to zero. In other words, the light incident on photoresist 110 in FIG. 1 has a spherical aberration of zero.
Systems for controlling spherical aberration of the light incident on a photoresist are disclosed in U.S. Pat. No. 5,432,587 to Nozue and U.S. Pat. No. 5,973,863 to Hatasawa et al. U.S. Pat. No. 5,636,000 to Ushida et al. discloses a projection lens system that controls environmental effects on the resulting projected image. U.S. Pat. No. 5,935,738 to Yasuzato et al. and U.S. Pat. No. 6,310,684 to Matsuura disclose methods to measure spherical aberration in projection lens systems.
While these prior art methods and apparatus have provided the capability to compensate for spherical aberration within photolithography exposing systems, they do not completely overcome the problem. This is due to the fact that the point for focusing a mask pattern is not selected to be at the surface of a photoresist layer, but is at some point within the photoresist layer near the middle of its cross-sectional thickness. By focusing the mask pattern in this fashion, a more uniform exposure over the entire cross-section of the photoresist is achieved. This, in turn, provides mask features with well-defined profiles. FIG. 3 shows a graph of mask feature profiles that result from focusing at different focal points within the photoresist. On the left, FIG. 3 shows that when the focus depth is too shallow, such as by focusing the light at or near the surface of the photoresist, the upper portion of the photoresist is overexposed, resulting in a mask feature profile 114A that is rounded-off and not thick enough to provide sufficient protection for the subsequent etching process. At a median focus depth, mask feature profile 114B has uniform sidewalls and maintains the thickness of the photoresist. At lower focus depths, the bottom portion of the photoresist is overexposed, resulting in undercutting as shown by mask feature profile 114C.
Because the light must pass through a portion of the photoresist layer to reach the focus point, however, the refractive index of the photoresist itself may induce spherical aberration. Referring to FIG. 4A, the origin of photoresist-induced spherical aberration is shown, where the photolithography process is adjusted in the conventional manner to compensate for spherical aberration from the optical system. Incident light 302, having a spherical aberration set at zero, arrives at photoresist 110 at various angles due to diffraction of light off of mask 108 (not shown, see FIG. 1). Incident light 302 is refracted due to the difference in the index of refraction between air 306 and photoresist 110. However, since incident light arrives at various different angles, it is refracted by different amounts in accordance with Snell's law and comes into focus at different depths within the photoresist 110. Thus, refraction by photoresist 110 causes the focal points for light arriving at different incident angles to be separated along a line, resulting in spherical aberration 308. This separation of the incident light 302 within photoresist 110 is the photoresist-induced spherical aberration. FIGS. 4A and 4B also illustrate how incident light 302 at differing angles would ideally unite accurately at a single focal point 304 (shown by broken lines in FIG. 4A) in the absence of refraction by photoresist 110.
Because the density of integrated circuitry patterns is constantly increasing to meet semiconductor technology requirements, photoresist-induced spherical aberration is progressively becoming a significant problem. Photoresist-induced spherical aberration causes the best focus to be a function of mask pattern density and feature size and pitch (spacing). This is due to the diffraction of light at different angles by various mask patterns. Reduced pitch gives higher diffraction. Thus, features like isolated lines will produce different focal points within the photoresist layer than dense line patterns. For each type of feature, there is a process window wherein light may be focused above or below the feature focal point and be within a tolerable range for forming an acceptable feature profile as shown in FIG. 3. Because the focal points for each feature are at different locations, the process windows for the various features will also be different. Therefore, to provide mask feature profiles that are within a tolerable range for all mask features having different sizes and pitches, it is necessary to focus the mask pattern at a point within the photoresist layer where portions of the process windows for each feature overlap, which is the overall process window for the mask pattern.
Because the photoresist-induced spherical aberration causes the focal points for the different mask features to be farther apart, there will be less overlap of their corresponding process windows. Therefore, if spherical aberration is not corrected or compensated for, the acceptable overall process window will be smaller, reducing process tolerances. Failure to correct for spherical aberration can cause misregistration in the resulting resist pattern and, if focus is poor, the resulting resist pattern is not as “clean” or well defined, as shown on the right and left sides of FIG. 3.
Accordingly, in order to improve the quality of patterns transferred to photoresists using photolithography, a need exists to reduce photoresist-induced spherical aberration.